In a typical cellular radio system, user equipment unit nodes (UEs) (also known as wireless terminals or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a RAN node, e.g., a radio base station (BS), which in some networks is also called a “NodeB” or enhanced NodeB “eNodeB.” A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase data rates and/or reliability of a wireless communication system. Performance may be improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. LTE-Advanced, for example, may support an 8-layer spatial multiplexing mode for 8 transmit antennas with possible channel dependent precoding. The spatial multiplexing mode is provided for high data rates in favorable channel conditions.
In a downlink from a base station transmitting from an antenna array over a MIMO channel to a UE, spatial multiplexing may thus allow the simultaneous transmission of multiple symbol streams over the same frequency. Stated in other words, multiple symbol streams may be transmitted from the base station to the UE over the same time/frequency resource element (TFRE). In such a system, precoding is preformed at the base station to distribute energy of a symbol vector (including the respective symbol streams) over the array of antennas, and the precoding may be performed using one of a plurality of precoder matrices selected to match characteristics of the MIMO downlink channel between the base station and UE.
Because MIMO downlink channel characteristics vary as a function of many factors including location of the UE relative to the base station, direction/speed of movement of the UE, etc., a large number of precoder matrices may be required to appropriately match the many possible variations of the MIMO downlink channel characteristics. Moreover, MIMO downlink channel characteristics may change during a communication between the base station and UE (e.g., due to movement of the UE). Accordingly, the UE may periodically/continuously monitor downlink channel characteristics during a communication to select/reselect a precoder (from a codebook of precoders) to be used by the base station for downlink transmissions, and the UE may transmit an identification of a selected precoder as needed and/or periodically to the base station over an uplink from the UE to the base station to provide closed loop precoder selection.
Because a relatively large number of precoder matrices may be defined/included in the codebook, the precoder identifications may be relatively lengthy to allow unique identification of each precoder matrix. Frequent transmission of precoder identifications over the uplink from the UE to the base station to communicate precoder selections may thus undesirable consume communication resources (e.g., bandwidth) in the uplink. Accordingly, there continues to exist a need in the art to provide improved closed loop precoder selection and related base stations and UEs.